With thousands of westerners fighting for islamist groups in Syria and Iraq, a foreign jihad no longer seems so distant, and officials fear that some of these fighters may eventually return radicalized by their experience—and ready to bring their war home

When Air Canada flight 8112 to Toronto took off from George Bush Intercontinental Airport in Houston on June 17, it was missing four passengers who had been scheduled to fly that afternoon. For several months a pair of undercover law-enforcement officials working for the FBI had been spending time with two of the passengers: a young Muslim convert named Michael Todd Wolfe and his wife Jordan Nicole Furr, according to an affidavit filed last week by an FBI agent. During that time, the affidavit says, Wolfe had discussed with the two undercover employees the possibility of joining an extremist Islamic group that the agent believed to be the Islamic State of Iraq and Greater Syria (ISIS), which is run by …